Why is it so hard to make a working toilet fit for space travel?

The space environment—microgravity, extreme temperatures and more—make it near-impossible to truly test a space toilet like Artemis II's ahead of launch, experts say

By Jackie Flynn Mogensen edited by Claire Cameron

NASA launched four astronauts on a pioneering journey around the moon—the Artemis II mission. Follow our coverage here.

The Artemis II mission was anything but easy. On April 1 NASA launched four astronauts inside a minivan-sized spacecraft on a record-breaking 10-day journey around the moon, bringing them safely back to Earth with incredible precision. It was a technological success for the agency as much as it was a symbolic one, inspiring a shared sense of awe for Earth and human capability. There was, however, one pesky problem: the astronauts couldn’t empty the toilet.

It wasn’t the toilet’s fault, according to the crew. Their Orion spacecraft’s loo—the Universal Waste Management System (UWMS)—was a “great piece of gear,” said NASA astronaut and Artemis II mission commander Reid Wiseman at a press conference after the crew had returned to Earth.

Indeed, it was something of a novelty: The moon-faring astronauts of the Apollo era had to relieve themselves in disposable bags. But now, 50-plus years later, Artemis astronauts could expect better. In the Artemis II mission, the Orion capsule was outfitted with an enclosed, 3D-printed titanium toilet, allowing the astronauts the luxury of pooping and peeing at the same time, and, crucially, a door to close for privacy. “I just want to say, 100 percent point-blank, that was a wonderful toilet. The toilet worked great,” Wiseman said.

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But where the crew ran into problems was with the urine vent line. The vent line appeared to clog about halfway through the 10-day mission, possibly because of the freezing of urine inside or debris from a chemical additive in the wastewater—it’s unclear, and NASA is investigating. “Teams will be assessing data from the Artemis II test flight in the coming weeks,” said Lauren Low, a public affairs specialist at NASA, in a statement to Scientific American.

The trouble, while somewhat amusing, raises a critical question: Why is it so difficult to flush urine from a toilet in space?

For one, there’s the absence of Earth’s gravity to hold everything down, says Mason Peck, a professor of astronautical engineering at Cornell University. “Fluid flows in unfamiliar ways in microgravity,” he says, and that makes it difficult to predict how liquids might behave. On Earth, we’re used to coffee resting in the bottom of our mugs, rain collecting in puddles—and urine traveling down a drain. “There are very few surprises about how water gets pulled toward the surface of the Earth because of gravity,” he says.

Without Earth’s gravity, fluid doesn’t necessarily flow “down.” Instead other forces take the wheel—surface tension, the shape of wastewater pipes, the motion of a spacecraft, and more can influence a fluid’s motion in space, which means it can be hard to design equipment that takes those subtleties into account. Introducing air to try and help push urine out through the plumbing could also create bubbles that cling to the pipes, says Pablo de León, a professor of space studies at the University of North Dakota, and this creates the potential for blockages.

To make matters trickier, space is a vacuum, and temperatures can fluctuate wildly. “You think it’s cold in Antarctica—it is colder in space,” Peck says. “You think it’s hot in the Sahara, in the Mojave Desert—it’s hotter in space. And that hot and cold can happen in the matter of minutes.” Researchers can simulate possible fluid dynamics in the space environment in labs on Earth ahead of launch, but “it’s basically never going to be perfect,” he says. “It’s amazing [the toilet] worked as well as it did, to be frank.”

It’s important to remember that the Artemis II toilet was a prototype. “This was the first time that humans used this particular system in space,” de León says. The International Space Station has a similar toilet setup, but it differs from the Orion capsule one in a crucial way: it recycles liquid waste for drinking water. Orion’s toilet, meanwhile, was designed to “vent,” or spew liquid waste into space.

“That is an interesting thing to see out the window,” Wiseman said at the same postsplashdown press conference. “It’s just like a billion little tiny flecks of ice heading out into deep space.”

The “good news,” Peck says, is that “we now learn from the experience.” Depending on the results of NASA’s toilet investigation, future changes to the design could include adding a heater or adjusting the flow of air or water in the venting lines—relatively minor tweaks in the grand scheme of things.

Ultimately, this mission was a successful test of the Orion spacecraft’s hardware, de León says. “If you go 400,000 kilometers and get back and the only issue you have is that the urine dump is not perfect, well, we have a good day.”